1. Field of the Invention
The present invention relates to an image forming apparatus which forms an image on a sheet, and more particularly to an image forming apparatus which can correct a displacement of a sheet in a direction crossing a sheet passage to prepare for image formation.
2. Description of the Related Art
A latent image corresponding to an original is formed on a photoreceptor, the latent image is visualized by attaching toner thereto, and a visualized toner image is transferred to the sheet in an image forming apparatus such as a copying machine, a printer, a facsimile, and a multi-function machine having functions thereof. Then, the toner image on the sheet is fixed by a fixing unit, and the sheet is discharged.
The sheet is usually stored in a sheet feeding tray, and a sheet is fed from the sheet feeding tray by a sheet transport unit, and is transported to an image forming unit for image formation. Moreover, the sheet on which the image is formed on a front surface side is turned over by a turnover and transport unit depending on necessity to circulate the sheet to the sheet transport unit, and the image formation is carried out on a rear surface side.
When the sheets are supplied to the sheet feeding tray, if pressing by a guide plate is not proper in a direction crossing sheet-passage, or the guide plate itself is loosen due to a problem in durability and others, and the sheets are consequently not pressed properly, the sheets are displaced in a direction crossing a sheet passage in the sheet feeding tray, which appears as a displacement upon sheet feeding. Moreover, also during the sheet transport after the sheet feed, the sheet is displaced in the sheet-passage crossing direction due to a vibration, a secular degradation of components, and the like, which appears as a displacement during the transport.
Thus, factors for the displacement corrected by the resist unit and the like are mainly the two factors which are the displacement during the sheet feed, and the displacement during the sheet transport.
For the above-mentioned displacement, a displacement correction mechanism to correct the displacement by moving a sheet, which is being transported, to a reference position in a direction crossing a sheet passage direction (sheet-passage crossing direction), and determines an image write position aligned with the reference position is proposed (refer to Japanese Patent Application Laid-open No. 2008-32913). The correction mechanism according to Japanese Patent Application Laid-Open No. 2008-32913 passes multiple sheets to automatically calculate the reference position thereby reducing a load imposed on the displacement correction, and is intended to increase productivity.
Moreover, according to Japanese Patent Application Laid-Open No. 2009-151230 carries out the displacement correction by means of an image shift, and the image shift does not calculate a correction quantity for an image with respect to a present sheet, but calculates the correction quantity from an average of detected quantities of the displacement of sheets up to one sheet before to enable the image shift.
As mentioned above, as the method of correcting the displacement of the sheet, there are the method of mechanically shifting the sheet depending on the correction quantity, and the method of shifting an image to be formed depending on the correction quantity.
The sheet shift by the first method needs to mechanically shift the sheet during a period corresponding to a sheet interval, and the interval between the sheets is increased a little so that the sheet shift operation is not delayed, which leads to a decrease in productivity. According to Japanese Patent Application Laid-Open No. 2008-32913, though the productivity is tried to be increased by combination with the image shift, the increase in productivity is limited, and the productivity is not sufficiently increased.
On the other hand, though image shift by the second method does not affect the productivity, the correction quantity needs to be calculated before the image formation starts. Therefore, the detection of the displacement quantity of the shift needs to be provided on a considerably more upstream side in the sheet transport direction compared with the sheet shift by the first method. If the transport after the detection extends over a long distance, a displacement may be generated during the transport, which leads to a decrease in accuracy of the displacement detection. According to Japanese Patent Application Laid-Open No. 2009-151230, the correction quantity is calculated from the average of the detected quantities of the displacement up to the one sheet before, the displacement quantity of the present sheet is not correctly recognized, and reliability of correction is inferior.